In announcing his candidacy for lieutenant governor, Corey Stew­art
touted his record of combating crime and illegal immigrants dur­ing his
years as chairman of the Prince William Board of County Supervisors.

“We have cut violent crime in half since we instituted what is
probably the nation’s toughest crackdown on illegal immigra­tion,”
Stewart, a Republican, said in an April 11 interview on Rich­mond’s NBC 12.
Stewart was referring to a law passed by Prince William super­visors
in October 2007. The law initially required county police to determine
the immigration status of anyone detained if there was probable cause to
suspect the person was not a legal U.S. resi­dent. The policy went
into effect in March 2008.

Because of concerns officers would
be accused of racial profil­ing, the board amended the law in April
2008 to require that police investigate the immigration status of
everyone arrested for violating state or local laws.

We checked
to see if violent crime has in fact dropped by half in Prince William
since the coun­ty started the crackdown and, if so, whether the
immigration law caused the decline.

The answers, we learned during many hours of research, are com­plex and often contradictory.

Statistical issues

Stewart’s campaign told us the candidate’s claim is based on
sta­tistics kept by the Prince William County Police Department.
An­nual reports compiled by the lo­cal police show 767 violent crimes —
which they define as murder, rape, robbery and aggravated as­sault —
reported to the depart­ment in 2006 compared with 402 in 2011. That’s a
drop of 47.6 percent.

But numbers in the Virginia State
Police’s annual Crime in Vir­ginia report show Prince William has
experienced a much more modest decrease in violent crime between those
years.

The State Police reports —which define violent crime as
murder, robbery, aggravated assault, forc­ible rape and “other forcible
sex crimes” — show only a 24 percent decline in the number of vio­lent
crimes between those years. That’s barely above the 22 percent drop off in violent crime experi­enced throughout Virginia from 2006 to 2011, according to State Police reports.

On the other hand, Prince Wil­liam significantly topped the combined
performance of its neighboring counties — Fau­quier, Loudoun, Stafford
and Fairfax — which saw a 27 percent increase in violent crime between
2006 and 2011.

For additional insight, we turned to the Federal
Bureau of Investiga­tion. Its statistics show the num­ber of violent
crimes reported by the Prince William County Police Department dropped
23 percent from 2006 to 2010, the latest year available. That’s slightly
above the 21 percent drop across Virginia, according to FBI data.
Prince William’s annual crime reports, meanwhile, show a 33.4 percent
drop in violent crime during that time frame.

Why are the numbers so different?

Jonathan Perok, a spokesman for the Prince William police, said the
county uses two different methods for recording crimes — an old summary
system for com­piling its annual county reports and an incident-based
system it uses when reporting its data to the state police.

Under the county’s system, the most serious offense in a criminal
incident is counted. For example, in an incident involving a murder and a
robbery, only the murder would be tallied.

But the county
reports a differ­ent set of statistics to the State Police, who count
all offenses in an incident when compiling their annual crime reports.

Adding to the confusion, the data sent to the state is in turn given
to the FBI, which converts it back to the old summary data for its
national report. Perok said the FBI’s converted numbers never match
Prince William’s original statistics.

The old summary system on which Prince William bases its reports has
been used nation­wide since the 1930s, according to the FBI. But law
enforcement agencies have been switching to incident-based reporting,
which offers more details.

Our confusion over the statis­tics
was shared by the co-authors of a 2010 study, sponsored by the
University of Virginia, examining the impact Prince William’s crack­down
on immigrants had on its crime rate.

“We ran into a very
similar situ­ation that was driving me abso­lutely crazy because we
were coming up with completely dif­ferent trends for similar time
pe­riods,” said Tim Carter, a report co-author and the head of James
Madison University’s sociology department.

The authors noted
the problem in their study, and ultimately re­lied on Prince William’s
police re­ports for much of their research.

Norman Westerberg,
program manager with the Virginia State Police, was puzzled about why
Prince William would use old summary reporting to tally crimes for its
annual reports. He said the incident based format is more comprehensive.

But Dana Schrad, executive director of the Virginia Associa­tion of
Chiefs of Police, said other local agencies throughout the commonwealth
still use the old summary reporting system. She said keeping data in the
old for­mat allows for historic compari­sons with crime data from
earlier years. Some police officials think the older system provides a
more accurate view of crime in their communities, she added.

When to start counting

We encountered another statis­tical problem with Stewart’s state­ment. He said there has been a
50 percent drop in violent crime since the policy was “instituted,”
meaning started.The law, as we’ve noted, didn’t pass until October 2007
and didn’t go into effect until March 2008.

Stewart gets near
his 50 percent claim by using 2006 county police statistics as his base
year to start measuring the decrease in violent crime.

But that’s more than a year be­fore the law was “instituted.”

If we use 2007 as the starting point, the violent crime drop was 35
percent between that year and 2011, according to the county’s annual
reports. State Police re­ports show violent crime in Prince William
decreased by 2 percent during that time. Violent crime in Virginia went
down 21 percent during that span.

The Stewart campaign said
2006 is a legitimate starting point be­cause it was the last full year
be­fore the crackdown came up for consideration. Public debate over the
measure began in mid-2007, they said, beginning the drop in violent
crime.

Cause and effect

Next we turn to the implication of Stewart’s statement — that the immigration law caused the de­crease in violent crime.

The UVa study, which the Stew­art campaign urged us to read, of­fers no definite answer to this part of the claim.

The report said violent crime in the county rose from 2004 to 2006 and
then fell during 2007 and 2008. The drop was largely due to a 36
percent decline in aggravated assaults.

The report said
aggravated as­saults really began falling in July 2007, when
controversy over the proposed policy began to rise.

“It’s probable, but by no means certain, that the drop in aggra­vated
assaults is attributable to the immigration policy that the county
adopted,”Tom Guterbock, one of the report’s authors, told us. “It’s a
very conspicuous drop. … Other counties [in northern Virginia] didn’t
see such a drop.”

But Guterbock, the director of the Center for
Survey Research at UVa, said other report co-authors were “less sure”
the law caused the decrease.

The report said, “On balance, our
conclusions about the policy’s impact on crime must be cautious due in
large part to the lack of his­torical data on crimes committed by illegal immigrants.”

Until the law went into effect, PrinceWilliam did not tally figures
for crimes in which the suspects were illegal immigrants. The re­port
said illegal immigrants, since the crackdown began, have com­mitted a
small to modest percent­age of crimes in Prince William, including
aggravated assaults.

The report raises other possible reasons
for the drop in crime, par­ticularly aggravated assaults. Im­migrants
may have become more fearful of reporting crimes once the law went into
effect, accord­ing to the study.

Our ruling

Stewart said violent crime in Prince William County has been cut in
half since the county “insti­tuted” its 2008 crackdown on im­migrants.
The claim only comes close to holding up when Stewart uses statistics
kept by the county police and starts his count with crime data from 2006
— more than a year before the crackdown began.

Data from the
state police — which uses a more compre­hensive formula than Prince
William to count violent crime — tells a different story. It shows only
an 24 percent drop in Prince William from 2006 to 2011 just above
Virginia’s average. FBI sta­tistics, based on reports from the county’s
police, show a 23 percent reduction in violent crime from 2006 to 2010,
slightly above Vir­ginia’s decline.
It’s important to note, however, that Prince William did outper­form
its neighboring Northern Virginia counties, which saw vio­lent crime
increase from 2006 to 2011, according to state police figures.

Stewart’s statement implies Prince William’s drop in violent crime was
caused by the crack­down. Experts who studied the impact of the law
came to no de­finitive conclusion on that point.